1. Field of the Invention
This invention relates generally to regulator systems for dynamoelectric machines and more particularly to apparatus for controlling the reactive volt-amperes in synchronous alternating current (a.c.) generators by controlling the excitation current in the field winding of the generators in such systems.
2. Description of the Prior Art
Reactive volt-ampere limiting has been applied to power systems since the early application of voltage regulators to synchronous dynamoelectric machines, such as, steam turbine generators, hydro-generators, and synchronous condensers.
Today, practically all regulators for dynamoelectric power systems are supplied with an underexcited reactive ampere limit. The purpose of the underexcited reactive ampere limit in the system is to prevent the a.c. machine from exceeding its steady-state stability limit (loss of synchronism with other machines in the power system due to system disturbances).
Most all power systems presently making use of the underexcited reactive ampere limit have circuits which utilize the a.c. machine terminal voltage and current for determining stability limit characteristics.
Two known prior art systems are documented in U.S. Pat. No. 2,478,623, entitled "Regulator For Synchronous Dynamoelectric Machines", issued to Selden B. Crary and Melville E. Hartman and assigned to the assignee of the present invention, and a paper entitled "Underexcited Reactive Ampere Limit For Modern Amplidyne Voltage Regulator", by A. S. Rubenstein and M. Temoshok, identified as paper 54-504 approved by the AIEE Committee on Technical Operations presented at the AIEE Fall General Meeting, Chicago, Illinois, Oct. 11-15, 1954.
Each of these references discloses, in one form or another, an underexcited reactive ampere limit circuit which obtains its inputs from the a.c. machine terminals (voltage and current). The function of the limit circuit is to detect an increase in the underexcited reactive current in the a.c. machine beyond a preset value, and to provide a signal to a voltage regulator or power supply means which acts to limit the increase in underexcited reactive current. This underexcited reactive ampere limit serves to prevent reduction of the a.c. machine field winding excitation current below a preset point. An increase in the underexcited reactive current, beyond a preset value, effectively causes the underexcited reactive ampere limit to readjust the voltage regulator or power supply means supplying current to the a.c. machine field winding to hold a higher machine voltage. In generators, the allowable maximum underexcited reactive current is generally varied as a function of the power output of the generator. The amount by which the excitation current is raised is proportional to the increase in the generator reactive volt-amperes.
While the prior art systems have served to substantially control the underexcited reactive current limit, they have performed that control by the establishment of limits in which the underexcited reactive limit can operate. That is, the limiting action (limit start point) starts when the underexcited reactive current reaches a preset value and acts to keep the reactive current within a range of values between the start point and a maximum value of underexcitation current.
While this operating in a range of values serves to effectively prevent the a.c. machine field excitation current from being lowered beyond that value required for machine stability to prevent pullout or loss of synchronism, it does not allow a high precise control of the reactive ampere limit which enhances overall machine performance under all operating conditions.
It is, therefore, desirable to provide an improved regulator for controlling dynamoelectric machines which accurately controls the value of the machine reactive current without the need of an established "start point" to control the underexcited reactive current limit.